This invention refers to an inverter or converter by current injection, provided with a circuit whose generating frequency for the alternating voltage is adjusted by the load""s resonant frequency, cycle to cycle, without lags, which permits avoiding power losses when transferring energy as a result of the variation in the load""s resonant frequency that necessarily occurs in all fixed frequency inverters, characterized as a frequency interlocking circuit, provided with at least one voltage scanner in the load, adapter and galvanic insulation, one lead circuit, one clipping circuit, one comparator circuit and one oscillator and coupling circuit.
This invention refers to an inverter or converter by current injection, provided with a circuit whose generating frequency for the alternating voltage is adjusted by the load""s resonant frequency, cycle to cycle, without lags, which permits avoiding power losses when transferring energy as a result of the variation in the load""s resonant frequency that necessarily occurs in all fixed frequency inverters.
In general, in the field of electrical engineering, the inverter or converter is the device, equipment or electric system that is able to convert continuous power (kW) into alternate power (kva).
There are different types of converters. Depending on the type of electric power that they supply, these are classified in four categories, according to the process they carry out:
a) AC/DC rectification, a process that converts alternate current into continuous current;
b) CC/CC conversion, that converts a continuous current into another that is also continuous but has different voltage characteristics or levels;
c) DC/AC inverter, conversion of continuous current into alternate, and
d) AC/AC conversion, a process that converts one alternate current into another that is also alternate but has different voltage and/or frequency characteristics.
The transformation from continuous voltage to alternate voltage is achieved by inverting the polarity of the source on the load by using interruption and connection devices.
A source of continuous voltage, see FIG. 1, in this case a battery is connected to a load using a set of four switches that act in pairs, connecting alternate polar voltage periodically. The load xe2x80x9cperceivesxe2x80x9d a voltage source in the shape of a square wave that alternates the value +Vcc and xe2x88x92Vcc. This is the principle of any inverter, a source of continuous power controlled by a group of switches that alternate the polarity in the load producing an alternated signal.
This is the operating principle of any inverter, a source of continuous power controlled by a set of switches that alternate the polarity in the load producing tension and alternated current in the load.
Present-day inverters or converters employ solid state elements, capable of controlling high powers that act as controlled electronic switches that periodically exchange the polarity of the continuous source on the load to a pre-assigned frequency, by means of electronic oscillatory circuits (interval timers).
The name resonant tank is given to a circuit (group of passive electric elements), formed by resistors, capacitances and inductances (RLC circuit) placed in a such a way that in order to inject current (parallel resonant) or applied voltage (series resonant) with a frequency known as resonant frequency, the impedance of the capacitance and inductance annul each other and remain limited only by the resistance of the circuit.
The circuit of the inverter that we wish to patent, refers exclusively to the case of parallel resonant tanks that are characterized because the capacitance is in parallel with the inductance and resistance as shown in FIG. 2.
Consequently, the parallel resonant circuits produce a great alternate current circulating between the coil and the condenser when they are in resonance, limited only by the series resistance, with a small real excitation current entering the tank.
This result is very often used to produce heating by magnetic induction, because according to the Ampere Circuital Law all intensity of alternate current produces a magnetic field around it that, in turn, induces voltages in any conductor that is near it. This is the principle whereby inductive heating is produced; internal voltages are induced in a metal conductor, in the presence of an alternate magnetic field, which cause currents to circulate that produce the heating due to the Joule effect.
All the present-day inversion systems for these applications of inductive heating work at a fixed frequency, normally within the range of 200 Hz to 10 kHz, designed to generate, by the injection of a continuous current, an alternating voltage of fixed frequency to the resonance of the tank assuming an invariability in time of the tank""s resonant frequency.
The specific construction of the single-phase inverters by current injection that exist in the market are manufactured using rapid power thyristors or more recently by GTO (gate turn-off). Both are solid-state devices able to rectify alternate current that conduct current in a single direction and whose conduction mode is to have a positive anode-cathode polarity and an electronic switch signal in their tripping gate.
To cut the conduction, the current must necessarily be zero, therefore a current must be injected that has the same magnitude but in the cathode-anode direction, by the voltage of the load of the tank itself (load switching) tripping the other pair of lead semiconductors with regard to the passing through zero of the voltage. Consequently, the lead time in the tripping depends on the frequency and is a very delicate value because faced with a variation in frequency the SCR (Silicon Controlled Rectifier) might not be turned of or it may be subjected to excessive voltage. All the above necessarily forces the circuits to be of a fixed frequency and higher than the resonant frequency.
The resonant inverters have a broad field of application in industry in general. They are used where a clean, rapid and efficient transfer of heat is required such as thermal treatments and fusion of metals.
The principle of energy transference is through the generation of an intense magnetic field produced by a high resonance current that circulates in the RLC tank.
The problem solved by this invention is to avoid power losses in the transfer of energy resulting from the variation of the load""s resonant frequency that is produced necessarily in all fixed frequency inverters and it also allows the inverter to work in resonant frequency with any load without needing special adjustments, within the range for which it was designed.
In effect, the tank""s resonant frequency (RLC circuit) depends only on the physical characteristics of its components, that is, resistance, inductance and capacitance. In the particular case of an oven, the resistance depends specially on its volume, the type of metal and its magnetic properties. The inductance will depend principally on its physical dimensions, number of windings, material to be heated or melted and temperature. On the other hand, normally, the capacitance is fixed with values that may be adjusted discretely (taps).
In particular, the temperature has an important effect on the coil""s resistance and reluctance. This is specially highlighted in the case of magnetic materials that, below the Curie temperature (760xc2x0 C.), have a magnetic permeability approximately 50 times greater than when they are above this temperature (magnetic permeability of the vacuum).
All the nonmagnetic metals such as copper, aluminum, certain steels, etc. present a permeability of dose to 1 or equal to 1 (permeability of the vacuum) and in these cases the reluctance varies little but the resistance, that increases as the temperature rises, does.
The coil""s inductance varies with the number of turns and the diameter. Consequently, when the inverter has a fixed frequency there is one and only one coil-condenser pair that will adjust to the frequency of the source. If the coil is changed then it becomes necessary to change the capacity in the condensers.
To summarize, fixed frequency inverters have the following limitations:
Variations of the inductance require the modification of the capacitance and vice versa.
The load does not always operate at resonant frequency.
The energy transfer power is not constant throughout the temperature.
A source cannot feed the different tanks without modifying its characteristics to adapt it to the frequency of the inverter.
The filling volume affects the resistance and thus the tank""s resonant frequency; therefore operating at half volume or partial load is inefficient.
The manner in which the load is filled influences the value of the inductance and therefore the resonant frequency.
The problem in the present-day technique that this invention solves is that, because of their present characteristics, existing inverters cannot be designed to operate at a variable xe2x80x9ccycle to cyclexe2x80x9d frequency as a result of the intrinsic characteristics of these power circuits that normally employ semiconductors of the thyristor or GTO (Gate Turn Off) type.
In effect, turning off the semiconductors requires an important current in the inverse direction, for which the same tension of the resonant tank is used, applied in the reverse manner by tripping the opposite branch of the inverter. This signal of conduction to the opposite branch must be advanced at the moment in which the tension is annulled to have an inverse tension that is sufficient to force the turning off of the thyristor. Otherwise, the SCR may continue conducting and short-circuit the tank If, because of a variation in the load, the frequency has to be lowered, it is very probable that the delay in the tripping of the opposite branch will mean that there will not be sufficient energy in the tank to turn it off. On the other hand, if the frequency increases, the conduction of the SCR will be very small and will produce strong oscillations in the inverter bridge.
Another important problem in the current technique solved by the invention is that the resonant inverters switched by the load require a STARTING POWER CIRCUIT that sends a pulse of energy to the resonant tank to start the oscillation and is not used after that. The scheme developed by the inverter of this invention does not require this auxiliary power circuit because it starts with the load directly.
To summarize, the present solutions for the resonant inverters switched by the load have two inconveniences for which the technique has not found a commercial solution:
They cannot work at variable frequencies cycle to cycle.
They require a start-up power circuit.
The principal disadvantages or problems of the present resonant inverters switched by the load are:
Load factor low when not transferring the power in condition of 100% of resonance for all points of operation.
The tank""s load must be constant.
Cannot work with different tanks without modifying its RCL parameters externally to adjust to the frequency of the source.
The cutting of the conduction of the semiconductors does not take place at the minimum tension point in the condensers.
They require more electronics and power components for the ignition circuit.